1. Field of the Invention
This invention relates to NMR spectroscopy. In a primary application it relates to measuring the amplitude of NMR spectral components with immunity to inhomogeneity.
2. Description of Prior Art
Magnetic resonance imaging systems currently provide excellent images of the large water line in the hydrogen spectrum. Images are also made of the fat or lipid line in the hydrogen spectrum. A popular method of making images of the lipid line is described in the publication by W. T. Dixon in Radiology, 153:189 (1984). Here data is obtained of both water plus fat and water minus fat, enabling the individual components to be separated.
The thusfar unrealized promise of magnetic resonance, however, is the mapping of the subtle spectroscopic components which can enable the diagnosis of disease through the measurement of important biochemical constituents. Thusfar this is not done on an imaging basis. For the most part, in vivo spectroscopy involves receiving signals representing one region of the anatomy and decomposing that signal, using a Fourier transform, into a frequency spectrum. The spectroscopist observes this spectrum and estimates the relative amplitude of the important components. Methods have recently been devised for moving the volume of observation around. One example is the paper by R. Ordidge in the Book of Abstracts of the 4th Annual Meeting of SMRM, p. 131 (1985).
The first effort at an imaging system which can preserve spectroscopic components is the 3DFT approach described in "In Vivo .sup.31 P NMR Imaging of Phosphor Metabolites," by J. Cuttaselgrove et al., Science 220:1170-1173 (1983). Here a sequence of excitations are used followed by sets of phase encoding pulses which place the resultant signal at a point in the two-dimensional k space. Thus all of k space is covered, given a sufficient number of excitations. Since no readout gradients are used during the time the signals are received, following the phase encoding, the spectroscopic information is preserved, using a 3DFT, with two spatial dimensions and one temporal dimension, the spectrum at each point is found. This approach has two practical problems. Firstly, the acquisition time is relatively long since each excitation represents a single point in k space. Secondly, as a result of inhomogeneity of the magnetic field, the demodulated spectrum at each voxel is shifted an arbitrary amount. This makes it very difficult to create spectroscopic images since the exact frequency reference has been lost.
In an effort to solve the problem of slow acquisition, A. Macovski introduced the use of time-varying gradients. Using periodic gradients, k-space, rather than a point, is covered during each excitation. This is described in the publication "Volumetric NMR Imaging with Time-Varying Gradients," by A. Macovski, J. of Magnetic Resonance in Medicine, 2:29-40, 1985. It is also described in U.S. patent application Ser. No. 603,333, by A. Macovski entitled "Simultaneous NMR Imaging System." This same basic concept of time-varying gradients for spectroscopy is also described in a publication by S. Matsui, K. Sekihara and H. Kohnu, "Spatially Resolved NMR Spectroscopy Using Phase-Modulated Spin-Echo Trains," J. of Magnetic Resonance in Medicine, 67:476-490, 1986.
Over and above the problem of high speed acquisition, is that of sensitivity. Since the metabolites of interest are relatively weak, they can present a significant SNR problem, especially when the acquisition is relatively rapid. The SNR problem can be significantly aided through the use of estimation theory as described in the publication by A. Macovski and D. Spielman, "In Vivo Spectroscopic Magnetic Resonance Imaging Using Estimation Theory," J. of MR in Medicine, 3:97-104, 1986. Here we make use of the a priori knowledge of the exact frequencies of each signal to estimate the amplitudes in the presence of noise. This provides a dramatic improvement over the prior art. However, inhomogeneity can provide an unknown frequency shift at each voxel.